I might be totally out in left field here (I am still using Tiger) but did you check your System Preferences -> Keyboard & Mouse and see if in the Keyboard Shortcuts tab, the key combo is activated or not? Or if you can assign the behaviour to this keyboard combo?

Its on the keyboard tab where it has a check box that says to "use all F1, F2 keys as regular function keys," uncheck that then go to the shortcuts tab and change them to whatever you want them to do by clicking the + sign and adding applications to keys.